If I was in SFO I would sign up for the AMEX Platinum in a heartbeat because of the Centurion Lounge (which is located in the UA Terminal).

As others have mentioned, if you can maximize the Uber + Airline Credits the $550 AF comes down to a reasonable $150 ($200 Airline Credit + $200 Uber Credit). $150 for Hilton Gold, Starwood Gold (and by association, Marriott Gold) plus Priority Pass, Delta Lounge and Centurion Lounge Access is definitely worth it.

If you are on the fence, Ameriprise has a fee-free version you can try out for the first year. You won't get a signup bonus, but it's good to try out the plat card for the first time.